Stone
Keep in mind I'm only referencing the parallel sayings in Matt./Luke that I listed in a previous post, not the entirety of the sayings at all.
If there is a historical Jesus, and if his counting depends on his having been a Jewish teacher, then this kind of cherrypicking-by-proxy is fatal to any hope for recovery of the historical Jesus who counts. Your poster-child 'unprecedented saying,' to love your enemies, is plainly depicted as arising through a routine rabbinical teaching practice: calling attention to antithetical verses, then a synthetic verse, and then commentary (a kind of "dialectical" reasoning, not unique to rabbis, but popular with them).
For example, your example,
Matthew 5: 43, offers this introduction to the "innovation,"
You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’
Those are clear references to
Leviticus 19: 18 (The love part), already identified as a component of the chief commandment in
Mark, at 12: 31 and 33, juxtaposed with
Psalms 139: 19-22 (the hate part). There is a superficial reconciliation, but one that is easily defeated. As
Luke articulates the problem, "Who is your neighbor?" (at 10:29, shown there as a continuation of the Marcan incident at 12: 31 ff, already mentioned.)
The synthetic verse is, as was explained in a previous post,
Proverbs 25: 21-22, and we can be confident that we have the right verse, because Jesus keeps the petty character of the sage's advice to treat neighbor and enemy alike. Behave this way for that will accomplish, "heaping coals on their head" originally, better spiritual pay-off than theirs in the new juxtapostion.
The incident viewed whole has some "ring of truth."
Mathhew doesn't depict Jesus saying "love your enemies" out of nowhere, but shows Jesus deriving the advice systematically from Jewish scripture, much as a rabbi might plausibly really have done. To strip away the parts of the speech which show Jesus' thought process promotes Jesus' brilliance (and an anachronistic Gentile-friendly view of his mission) at the expense of his historical plausibility.